This invention is concerned with a process for the production of shaped-bodies and semi-finished products from polyolefins crosslinkable by high energy radiation.
Because of their good mechanical and electrical properties, thermoplastic polyolefins are used in many areas of technology. In many cases, however, the plastics used no longer meet today's increased requirements.
For some years now it has been possible to crosslink ethylene homo- and co-polymers such as ethylene-vinyl acetate (EVA), ethylene-propylene rubber (EPM) or ethylene-propylenediene terpolymers (EPDM) more or less intensively depending on the respective crosslinking method. Such crosslinkage is accompanied primarily by greater thermostability, reduced solubility in organic solvents, and increased resistance to mechanical stresses. The excellent dielectric properties of the polymers are not impaired by the crosslinking process or only insignificantly so. These improvements of the properties caused by crosslinking open up new technical applications for these materials.
The crosslinkage of polyolefins by action of high-energy radiation, e.g. beta, gamma or x-rays, is known. Sources of such radiation are electron accelerators, x-ray installations, radioactive isotopes, and nuclear reactors.
For a technically sufficient crosslinkage of low density polyethylene (LDPE), for example, a radiation dose of 200 - 300 kJ/kg (= 20-30 Mrd) must be applied. Since the required energy (kJ/kg) is a highly relevant factor in the cost analysis of any technical process based on radiation crosslinking, one endeavors to lower the dose required for technically sufficient crosslinking by additional measures. For example, according to German Offenlegungsschriften 1,544,804 and 1,544,805, polyfunctional olefin-unsaturated, i.e. ethylenically unsaturated monomeric compounds which act as crosslinking or vulcanization accelerators are used to lower the dose. The polyfunctional ethylenically unsaturated monomers which are used belong to the class of diacrylic and dimethacrylic acid esters of mono-, di-, tri- and tetra-ethylene glycol, as well as their vinyl and allyl esters; divinyl compounds such as divinyl benzene or diethylene-glycol divinyl ether, diallyl esters of maleic acid or malonic acid, diallyl compounds of pentaerythritol, triallyl cyanurate and related homologs and mixtures thereof.
Although a reduction of the radiation dose required for a technically sufficient crosslinkage can be brought about by addition of such coagents, i.e. polyfunctional olefinic monomers to the polymer before crosslinking, this method has a number of considerable disadvantages.
The reasons for these disadvantages lie particularly in the chemical nature of the monomers, i.e. the coagents. The above-mentioned polyfunctional olefin-unsaturated monomers are all polar compounds. They are largely insoluble in polyolefins of nonpolar structure, and consequently they gradually migrate from the as yet uncrosslinked polymer to the polymer surface where they appear as small droplets. This phenomenon is known as sweating. This sweating effect is visible already a few hours after the shaping and even at coagent concentrations of less than 1 %, and it becomes more and more evident with increasing coagent content. This is even more significant since it is known that with increasing content of the coagent the radiation dose required for effective crosslinkage decreases.
The result is that a shaped body not yet crosslinked, produced for example by injection molding or by extrusion of a mixture, even if properly stabilized, of polyolefin with a polyfunctional olefin-unsaturated monomer, has little storage stability until it is crosslinked with high-energy radiation. This fact proves extremely disadvantageous especially if crosslinking is carried out separately in time from the shaping, as is the case in an intermittent process. During the storage time, due to continuous reduction of the coagent concentration in the polyolefin, the mixture ratio and hence also the energy required for radiation crosslinking changes. This leads to unpredictable results of the crosslinking process and effects the properties of the finished, shaped bodies.
The coagents, i.e. the olefinic monomers generally used, even in liquid or solid form, are usually skin-irritants and are somewhat toxic substances which adversely affects the handling of the non-crosslinked shaped parts.